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Next time you flush a toilet in Whangarei, you could be helping to save ratepayer money and enlighten local farmers to a better way of dealing with effluent.

A new unit at the city's wastewater treatment plant will use methane - a byproduct of the sewage treatment process - to generate electricity. This power would be used to run other processes at the plant, said Whangarei District Council waste and drainage operations engineer Adam Twose.

The unit would also serve as a demonstration project for Northland farmers wanting to put similar technology to use with animal effluent.

"The electricity produced won't be enough to export to the grid because this site [the treatment plant] uses about 250 to 300 kilowatts all the time," Mr Twose said. "This process is only going to generate about 70 kilowatts, but that will save the council about $60,000 worth of electricity a year."

The WDC's Kioreroa Rd plant would soon see the installation of a General Motors 8-litre V8 engine next to its sewage digestors. The engine is similar to what can be found inside many American muscle cars and would form the basis of a co-generation unit, which produces useful electricity as well as the heat needed to "cook" sewage sludge as part of the treatment process.

"We cook the sludge at 37C for about 30 days, allowing the solids to break down," Mr Twose said. After this, the sewage sludge was dried and taken to landfill.

Mr Twose said sludge was now cooked using boilers, which ran off the methane this process produced - a self-contained process.

But about 57cu m of wasted methane was flared off hourly.

"Effectively, the unit we're putting in should stop all of that surfacing," Mr Twose said. "Then there will be a hot water jacket around [the engine] which collects heat to do what the boilers are doing at the moment. Instead of wasting the gas, we're getting electricity."

The WDC project, expected to cost about $200,000, was going ahead with a $79,600 grant from the Energy Efficiency Conservation Authority (EECA), meaning the project would have paid ratepayers back in less than three years.

Normally, technology bound for humans was first tested on animals. In this case, the opposite was true. EECA was keen on the project as farmers who collected their effluent in a tank could replicate the idea on a smaller scale.

"EECA will fund this so long as we're willing to open it up and let people come down and have a look. It's a pilot plant for the whole of Northland," Mr Twose said.

The plant treats the wastewater of more than 55,000 residents from Urquharts Bay to Otaika Valley - up to 21,000cu m a day. It could also treat an additional 100,000cu m during periods of heavy rainfall.